Schistosome infections are studied quantitatively in man and experimental animals. Schistosome japonicum infections in rabbits regularly produced Symmers' clay pipestem fibrosis of the liver, but this was often accompanied by a mixed macronodular-micronodular cirrhsis. Morphologic fibrosis and collagen content decreased between 30 and 50 weeks of infection. The size of hepatic granulomas around eggs remained nearly constant, but several factors combined to drastically reduce the number of eggs reaching the liver. Hepatic fibrosis was similar in rabbits infected with Japaneses and Philippine strains of the parasite. The passage of eggs in the feces parallelled the intensity of infection in rabbits infected with the Japanese, but not the Philippine, worm strain, apparently because most eggs were not deposited near mucosal surfaces by the latter.